Open-gain trans-impedance amplifier with programmable input impedance

ABSTRACT

In at least same examples, a communication device includes a photo-diode to convert an optical signal into an electrical current and an open-gain trans-impedance amplifier to amplify the electrical current. The communication device also includes a transmission line between the photo-diode and the open-gain trans-impedance amplifier. The open-gain trans-impedance amplifier includes a programmable input impedance that has been matched to an impedance of the transmission line.

BACKGROUND

For optical communications, the input optical pulse from the opticalfiber is received and converted into electrical current through asphoto-diode. A trans-impedance amplifier (TIA) may then be employed toconvert input current into voltage output. Since the electrical currentoutput by the photo-diode is very small, (e.g., on the order of 20 μA),the TIA is placed next to the photo-diode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of illustrative examples of the disclosure,reference will now be made to the accompanying drawings in which:

FIG. 1 shows a communication device in accordance with an example of thedisclosure;

FIG. 2 shows an arrangement of communication device components inaccordance with an example of the disclosure;

FIG. 3 shows a programmable trans-impedance amplifier (PTIA)architecture in accordance with an example of the disclosure;

FIG. 4 shows an output signal of an open-gain PTIA in accordance with anexample of the disclosure;

FIG. 5 shows an eye diagram corresponding to the output signal of FIG. 4in accordance with an example of the disclosure;

FIG. 6 shows an analysis of the eye diagram of FIG. 5 in accordance withan example of the disclosure;

FIG. 7 shows a computer system in accordance with an example of thedisclosure; and

FIG. 8 shows a method in accordance with an example of the disclosure.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, computer companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . ”. Also, the term “couple” or “couples” isintended to mean either an indirect, direct, optical or wirelesselectrical connection. Thus, if a first device couples to a seconddevice, that connection may be through a direct electrical connection,through an indirect electrical connection via other devices andconnections, through an optical electrical connection, or through awireless electrical connection.

DETAILED DESCRIPTION

Due to imperfect integrated circuit manufacturing processes, the inputimpedance of a trans-impedance amplifier (TIA) varies regardless ofefforts to design for a particular input impedance. Also transmissionline impedance may vary due to variations and imperfections in thematerials/designs for transmission lines. If the TIA input impedancedoes not match the transmission line impedance, the input signal will bedistorted due to reflection noise. In an optical communicationsapplication, the output current from a photo-diode is very small (e.g.,20 μA to 70 μA) and any signal distortion makes it difficult orimpossible to recapture the data. Instead of placing a TIA close to thephoto-diode to avoid signal distortion to the very small current outputfrom a photo-diode, disclosed examples use a TIA with a programmableinput impedance that is adjusted to match a transmission line impedance.Further, in some examples, MOSFET transistors may be used rather thanbipolar devices.

A TIA with a programmable input impedance is referred to herein as aprogrammable TIA (PTIA). In at least some disclosed examples, anopen-gain PTIA is part of a communication device that receives opticalsignals and processes corresponding electrical signals. In thecommunication device, a photo-diode converts optical signals toelectrical current. The electrical current from the photo-diode isreceived by the open-gain PTIA via a transmission line. The transmissionline between the photo-diode and the open-gain PTIA is due to, forexample, the open-gain PTIA being on integrated circuit (IC) chip thatis separate from the photo-diode. In some examples, the open-gain PTIAis implemented on an IC chip with other receiver components to reduceoverall cost, power, and latency requirements for the receiver. With theopen-gain PTIA separated from the photo-diode by a transmission line,the impedance of the transmission line and other noise coupling issuesneed to be addressed. The proposed open-gain PTIA resolves signalreflection problems due to non-matched transmission line impedance andopen-gain PTIA input impedance by adjustment of the open-gain PTIA inputimpedance to match the impedance of the transmission line. Adjustmentsto the input impedance of the open-gain PTIA may occur either throughpost-silicon calibration (before the chip with the open-gain PTIA isinstalled in a consumer communication device) or through systemprogramming (after the chip with the open-gain PTIA is installed in aconsumer communication device). In either case, the adjustments may bebased on an analysis of the output signal for the open-gain PTIA. Thedisclosed PTIA examples replace the existing technique of placing a TIAnext to the photo-diode to avoid signal distortion to the very smallcurrent output from the photo-diode.

FIG. 1 shows a communication device 100 in accordance with an example ofthe disclosure. The communication device 100 corresponds to an end-node(data sink) or routing device of an optical communication network. Asshown, the communication device 100 comprises a photo-diode 102 thatreceives an optical signal and outputs a corresponding electricalcurrent. The electrical current is propagated via a lossy transmissionline 104 (with impedance=Z0) to an open-gain PTIA 106. The open-gainPTIA 106 converts the electrical current received from the photo-diode102 via the lossy transmission line 104 to a voltage. As disclosedherein, the input impedance of the open-gain PTIA 106 is adjusted tomatch to an impedance of the transmission line.

In some embodiments, input impedance of the open-gain PTIA 106 isadjusted based on a control signal to a variable resistor. The controlsignal is based on analysis of an output signal of the open-gain PTIA106 either before or after the open gain PTIA 106 is installed in thecommunication device 100.

In some examples, the open-gain PTIA 106 comprises four amplifierstages, including a common gate amplifier stage followed by two commonsource amplifier stages. Further, the open-gain PTIA 106 may comprise alow-pass filter to feed the common mode signal as bias control for thecommon gate amplifier of the open-gain PTIA 106. Furthermore, thiscommon mode signal can generate a common mode reference voltage for thePTIA output. Therefore, low-pass filtering may be omitted for the outputsignal of the open-gain PTIA 106. In some examples, the open-gain PTIA106 is part of an integrated circuit that is separate from thephoto-diode 102 and the transmission line 104. For example, thephoto-diode 102 may be pan of an optical module. Meanwhile, thetransmission line 104 may correspond to semiconductor packaging and/or aconductive trace between an integrated circuit with the open-gain PTIA106 and the photo-diode 102.

To summarize, for the open-gain PTIA 106, the first amplifier stageprovides gain stage and voltage level shifting, while the second andthird amplifier stages provide further gain stage. The fourth amplifierstage provides voltage level shifting for the output signal and thecommon mode signal. A low-pass filter is used to feed back the commonmode signal to the common gate stage and output common mode referencestage. The programmable input impedance is based on equivalentresistance of R_term and (1 g_(m)1) in parallel, where R_term is avariable resistor value and g_(m)1 is the transconductance of thetransistor for the first amplifier stage. The value for R_term may bedetermined, for example, during a wafer test in which the PTIA inputvoltage is measured with zero current applied and also with a smallcurrent applied to the TIA input. The voltage difference is comparedwith a target voltage (V=I*Z0), where Z is the transmission lineimpedance to be matched. If the measured voltage value is higher thanthe target voltage, R_term should be reduced. Alternatively, if themeasured voltage is lower than the target voltage, R_term should beincreased. Another way to determine the value of R_term is to analyzethe eye-opening corresponding to the output of the open-gain PTIA 106,and to sweep across the available values for R_term (e.g., from low tohigh) until an optimal setting is determined.

FIG. 2 shows an arrangement 200 of communication device components inaccordance with an example of the disclosure. Without limitation toother examples, the arrangement 200 may be used for the communicationdevice 100 of FIG. 1. In the arrangement 200, various components areshown to be part of an optical module 202, a package 210, and a receiverchip 220. More specifically, the optical module 202 comprises acapacitor 204 and a photo-diode 102 as described for the communicationdevice 100 of FIG. 1. Further, the package 210 comprises a referenceground 214 and a lossy transmission line 104 as described for thecommunication device 100 of FIG. 1. Further, the receiver chip 220comprises various components as well as an op PTIA 106 as described forthe communication device 100 of FIG. 1.

As shown, the open-gain PTIA 106 receives its input from the lossytransmission line 104 and also receives the reference ground 214 viapackage 210. The receiver chip 220 also comprises a voltage reference230, a voltage regulator 226, and a capacitor 228 to provide a currentreturn path to the open-gain PTIA 106. The input impedance of theopen-gain PTIA 106 is adjustable by impedance controller 224, whichprovides a control signal to the open-gain PTIA 106. The control signalfrom the impedance controller 224 is based on instructions orinformation corresponding to the result of analyzing the output signalof the open-gain PTIA 106. As shown, the open-gain PTIA 106 outputs acommon mode signal and an output signal to receiver circuitry 234 of thereceiver chip 220. The receiver circuitry 234 may perform, for example,various data recovery operations.

FIG. 3 shows an open gain PTIA architecture 300 in accordance with anexample of the disclosure. As shown, the open-gain PTIA architecture 300comprises transistors M1-M5 with respective drain-side resistors R1-R5between a high_supply_reference and transistors M1-M5. In the open-gainPTIA architecture 300, a variable resistor (R_term) is placed between alow-supply reference and a source of transistor M1. The variableresistor R_term is controlled by a control signal (CTRL_(r) _(—)_(term)) as described herein. In the open-gain PTIA architecture 300, afeedback resistor (R_fb) and capacitor C3 are placed between a drain oftransistor M3 and the low_supply_reference. The signal between feedbackresistor R_fb and capacitor C3 is supplied to the gates of M1 and M5. Inthe open-gain PTIA architecture 300, capacitor CI is placed between asource of M4 and the high_supply_reference.

The open-gain PTIA architecture 300 of FIG. 3 corresponds to a fourstage amplifier. More specifically, M1 operates as a common gateamplifier and provides gain stage and voltage level shifting. Meanwhile,M2 and M3 operate as a common source amplifier to provide further gainstage. Further, M4 and M5 operate as a last stage common sourceamplifier to provide voltage level shifting for the output signal andcommon mode signal respectively. The feedback resistor R_fb andcapacitor C3 operate as a low-pass filter to provide a common modesignal back into the gate of M1. In this manner, the transistors M1-M5are placed in their optimal operation points. Further, voltage and inputswing variations are prevents while M1 operates in its saturation regionto have a high 1/gm value.

In the open-gain PTIA architecture 300, the variable resistor R_termcorresponds to a programmable passive resistor that is adjusted so thatthe PTIA input impedance matches a transmission line output impedance.The matching may be achieved by adjusting the resistor R_term directlywhile keeping the 1/gm large to have small impact on the inputimpedance. Further, by matching R4=R5 and M4=M5, the common mode levelhas been shifted to the proper level such that it matches the output.Therefore, additional low-pass filtering is not required for the output.

With the open-gain PTIA architecture 300, programmable input impedanceadjustment is utilized to match the transmission line impedance andreduce reflection. Further, the open-gain PTIA architecture 300 utilizesa common mode voltage feedback structure to tolerate high process andvoltage variation as well as swing control. Further, the open-gain PTIAarchitecture 300 does not use any inductor or cascading structure andthus size is reduced and low voltage operation is possible.

Without limitation to other examples, various values for the open-gainPTIA architecture 300 are provided herein. For example, the inputimpedance R_(ptia)=R_term/1 g_(m)1), Further, 1 g_(m)1 >20×Rterm suchthat R_(ptia)=R_term, which is trimable by Rtrim_cntl so thatR_(ptia)=Z0 (the transmission line impedance). Further, M4=M5 and R4=R5to provide level shifting and common mode output. Further, M1=6μ/80 n,and M3=M4=M5=4.2μ/40 n. Further, R1=3.2 kΩ, R2=1.6 kΩ, and R3=R4=R5=1kΩ. Further, R_term varies between approximately 40Ω to 100Ω to match toZ0=75Ω. Further, R_fb=25 kΩ, C3=12 pF, and C1=200 pF. Further, thephoto_current input is approximately 20 μA to 100 μA.

FIG. 4 shows an output signal 400 of an open-gain PTIA in accordancewith an example of the disclosure, As shown, the output signal 400ranges between 660×10⁻³ to 760×10⁻³ volts (0.66 to 0.76 volts) during atime period from 346×10⁻⁹ to 354×10⁻⁹ (approximately 0.1 μs).

FIG. 5 shows an eye diagram 500 corresponding to the output signal 400of FIG. 4. In the eye diagram 500, the quality of the output signal 400can be assessed as signal transitions occur between 660×10⁻³ to 760×10⁻³volts (0.66 to 0.76 volts) during a time window of 100×10⁻¹² seconds(100 picoseconds).

FIG. 6 shows an analysis 600 of the eye diagram 500 of FIG. 5 inaccordance with an example of the disclosure. In the analysis 600, thesignal transitions between 660×10⁻³ to 760×10⁻³ volts (0.66 to 0.76volts) during a time window of 100×10⁻¹² seconds (100 picoseconds) areshown as described for the eye diagram 600 of FIG. 5. In addition,analysis 600 shows an offset sampling with V_(ref)=0.725 V and a datasampling with V_(ref)=0.71 V. The error in the output signal 400 can bedetermined by XORing the offset sampling and the data sampling.Thereafter, the error can be reduced by adjustment of the inputimpedance of the open-gain PTIA 106 as described herein.

FIG. 7 shows a computer system 700 in accordance with an example of thedisclosure. The computer system 700 may correspond to part of anend-node (data sink) or routing device of an optical communicationnetwork. In other words, the components shown for the computer system700 may be part of a communication device 100 as described for FIG. 1.After reception of data from an optical communication network, thecomputer system 700 may store, process, and execute the received data.

As shown, the computer system 700 includes a processor 702 (which may bereferred to as a central processor unit or CPU) that is in communicationwith memory devices including secondary storage 704, read only memory(ROM) 706, random access memory (RAM) 708, input/output (I/O) devices710, and network connectivity devices 712. The processor 702 may beimplemented as one or more CPU chips.

It is understood that by programming and/or loading executableinstructions onto the computer system 700, at least one of the CPU 702,the RAM 708, and the ROM 706 are changed, transforming the computersystem 700 in part into a particular machine or apparatus having thenovel functionality taught by the present disclosure. It is fundamentalto the electrical engineering and software engineering arts thatfunctionality that can be implemented by loading executable softwareinto a computer can be converted to a hardware implementation bywell-known design rules. Decisions between implementing a concept insoftware versus hardware typically hinge on considerations of stabilityof the design and numbers of units to be produced rather than any issuesinvolved in translating from the software domain to the hardware domain.Generally, a design that is still subject to frequent change may bepreferred to be implemented in software, because re-spinning a hardwareimplementation is more expensive than re-spinning a software design.Generally, a design that is stable that will be produced in large volumemay be preferred to be implemented in hardware, for example in anapplication specific integrated circuit (ASIC), because for largeproduction runs the hardware implementation may be less expensive thanthe software implementation. Often a design may be developed and testedin a software form and later transformed, by well-known design rules, toan equivalent hardware implementation in an application specificintegrated circuit that hardwires the instructions of the software. Inthe same manner as a machine controlled by a new ASIC is a particularmachine or apparatus, likewise a computer that has been programmedand/or loaded with executable instructions may be viewed as a particularmachine or apparatus.

The secondary storage 704 is typically comprised of one or more diskdrives or tape drives and is used for non-volatile storage of data andas an over-flow data storage device if RAM 708 is not large enough tohold all working data. Secondary storage 704 may be used to storeprograms which are loaded into RAM 708 when such programs are selectedfor execution. The ROM 706 is used to store instructions and perhapsdata which are read during program execution. ROM 706 is a non-volatilememory device which typically has a small memory capacity relative tothe larger memory capacity of secondary storage 704. The RAM 708 is usedto store volatile data and perhaps to store instructions. Access to bothROM 706 and RAM 708 is typically faster than to secondary storage 704.The secondary storage 704, the RAM 708, and/or the ROM 706 may bereferred to in some contexts as computer readable storage media and/ornon-transitory computer readable media.

I/O devices 710 may include printers, video monitors, liquid crystaldisplays (LCDs), touch screen displays, keyboards, keypads, switches,mice, track balls, voice recognizers, card readers, paper tape readers,or other well-known input devices.

The network connectivity devices 712 may take the form of modems, modembanks, Ethernet cards, universal serial bus (USB) interface cards,serial interfaces, token ring cards, fiber distributed data interface(FDDI) cards, wireless local area network (WLAN) cards, radiotransceiver cards such as code division multiple access (CDMA), globalsystem for mobile communications (GSM), long-term evolution (LTE),worldwide interoperability for microwave access (WiMAX), and/or otherair interface protocol radio transceiver cards, and other well-knownnetwork devices. As described herein, the computer system 700 may bepart of an optical communication network. In such case, the networkconnectivity devices 712 support optical communication techniques. Thesenetwork connectivity devices 712 may enable the processor 702 tocommunicate with the Internet or one or more intranets. With such anetwork connection, it is contemplated that the processor 702 mightreceive information from the network, or might output information to thenetwork in the course of performing the above-described method steps.Such information, which is often represented as a sequence ofinstructions to be executed using processor 702, may be received fromand outputted to the network, for example, in the form of a computerdata signal embodied in a carrier wave. As shown, at least one ofnetwork connectivity devices 712 may comprise the receiver chip 220and/or the open-gain PTIA 106 described herein.

Such information, which may include data or instructions to be executedusing processor 702 for example, may be received from and outputted tothe network, for example, in the form of a computer data baseband signalor signal embodied in a carrier wave. The baseband signal or signalembedded in the carrier wave, or other types of signals currently usedor hereafter developed, may be generated according to several methodswell known to one skilled in the art. The baseband signal and/or signalembedded in the carrier wave may be referred to in some contexts as atransitory signal.

The processor 702 executes instructions, codes, computer programs,scripts which it accesses from hard disk, floppy disk, optical disk(these various disk based systems may all be considered secondarystorage 704), ROM 706, RAM 708, or the network connectivity devices 712.While only one processor 702 is shown, multiple processors may bepresent. Thus, while instructions may be discussed as executed by aprocessor, the instructions may be executed simultaneously, serially, orotherwise executed by one or multiple processors. Instructions, codes,computer programs, scripts, and/or data that may be accessed from thesecondary storage 704, for example, hard drives, floppy disks, opticaldisks, and/or other device, the ROM 706, and/or the RAM 708 may bereferred to in some contexts as non-transitory instructions and/ornon-transitory information.

In an embodiment, the computer system 700 may comprise two or morecomputers in communication with each other that collaborate to perform atask. For example, but not by way of limitation, an application may bepartitioned in such a way as to permit concurrent and/or parallelprocessing of the instructions of the application. Alternatively, thedata processed by the application may be partitioned in such a way as topermit concurrent and/or parallel processing of different portions of adata set by the two or more computers. In an embodiment, virtualizationsoftware may be employed by the computer system 700 to provide thefunctionality of a number of servers that is not directly bound to thenumber of computers in the computer system 700. For example,virtualization software may provide twenty virtual servers on fourphysical computers. In an embodiment, the functionality disclosed abovemay be provided by executing the application and/or applications in acloud computing environment. Cloud computing may comprise providingcomputing services via a network connection using dynamically scalablecomputing resources. Cloud computing may be supported, at least in part,by virtualization software. A cloud computing environment may beestablished by an enterprise and/or may be hired on an as-needed basisfrom a third party provider. Some cloud computing environments maycomprise cloud computing resources owned and operated by the enterpriseas well as cloud computing resources hired and/or leased from a thirdparty provider.

FIG. 8 shows a method 800 in accordance with an example of thedisclosure. The method 800 may be performed, for example, by anopen-gain PTIA as described herein. As shown, the method 800 comprisesreceiving an electrical current from a photo-diode via a transmissionline (block 802). The electrical current is converted to a voltage basedon a programmable input impedance of the open-gain PTIA at block 804,where the programmable input impedance of the open-gain PTIA is adjustedto match an impedance of the transmission line. As an example, adjustingthe programmable input impedance of the open-gain PTIA may be based on acontrol signal to a variable resistor, where the control signal is setaccording to a signal quality analysis of an output signal of theopen-gain PTIA.

To convert the electrical current voltage, the open-gain PTIA mayoperate a common gate amplification stage followed by two common sourceamplification stages. In some examples, converting the electricalcurrent to a voltage also may comprise voltage level shifting at a laststage of amplification for an output signal and for a common mode signalof the open-gain trans-impedance amplifier. Converting the electricalcurrent to a voltage also may comprise low-pass filtering the commonmode signal fed back to a first amplifier stage of the open-gain PTIA,and omitting low-pass filtering for the output signal of the PTIA. Formethod 800, the open-gain PTIA is part of an integrated circuit that isseparate from the photo-diode and the transmission line. The method 800also may comprise performing any other operations for setting up oroperating an open-gain PTIA separated from a photo-diode via atransmission line as described herein.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted or not implemented.

Also, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as directly coupled or communicating witheach other may be indirectly coupled or communicating through someinterface, device, or intermediate component, whether electrically,mechanically, or otherwise. Other examples of changes, substitutions,and alterations are ascertainable by one skilled in the art and could bemade without departing from the spirit and scope disclosed herein.

What is claimed is:
 1. A communication device, comprising: a photo-diodeto convert an optical signal into an electrical current; an open-gaintrans-impedance amplifier to convert the electrical current to avoltage; and a transmission line between the photo-diode and theopen-gain trans-impedance amplifier, wherein the open-gaintrans-impedance amplifier comprises a programmable input impedance thathas been matched to an impedance of the transmission line.
 2. Thecommunication device of claim 1, wherein the programmable inputimpedance is adjusted based on a control signal to a variable resistor,and wherein the control signal is based on analysis of an output signalof the open-gain trans-impedance amplifier.
 3. The communication deviceof claim 1, wherein the open-gain trans-impedance amplifier comprisesfour amplifier stages.
 4. The communication device of claim 1, whereinthe open-gain trans-impedance amplifier comprises a common gateamplifier stage followed by two common source amplifier stages.
 5. Thecommunication device of claim 1, wherein a last stage of the open-gaintrans-impedance amplifier provides voltage level shifting for an outputsignal and for a common mode signal of the open-gain trans-impedanceamplifier.
 6. The communication device of claim 5, wherein the open-gaintrans-impedance amplifier comprises a low-pass filter to feed the commonmode signal to a first amplifier stage of the open-gain trans-impedanceamplifier, and wherein low-pass filtering is omitted for the outputsignal of the open-gain trans-impedance amplifier.
 7. The communicationdevice of claim 1, wherein the open-gain trans-impedance amplifier ispart of an integrated circuit that is separate from the photo-diode andthe transmission line.
 8. A method for an open-gain trans-impedanceamplifier, comprising: receiving an electrical current from aphoto-diode via a transmission line; and converting the electricalcurrent to a voltage based on a programmable input impedance of theopen-gain trans-impedance amplifier, wherein the programmable inputimpedance is adjusted to match an impedance of the transmission line,wherein the open-gain trans-impedance amplifier is part of an integratedcircuit that is separate from the photo-diode and the transmission line.9. The method of claim 8, further comprising: adjusting the programmableinput impedance based on a control signal to a variable resistor,wherein the control signal is set according to a signal quality analysisof an output signal of the open-gain trans-impedance amplifier.
 10. Themethod of claim 8, wherein converting the electrical current to avoltage comprises operating a common gate amplification stage followedby two common source amplification stages.
 11. The method of claim 8,wherein converting the electrical current to a voltage comprises voltagelevel shifting at a last stage of amplification for an output signal andfor a common mode signal of the open-gain trans-impedance amplifier. 12.The method of claim 11, wherein converting the electrical current to avoltage comprises low-pass filtering the common mode signal fed back tobias voltage of a common gate amplifier stage, and omitting low-passfiltering for the output signal of the open-gain trans-impedanceamplifier.
 13. An integrated circuit chip, comprising: an open-gaintrans-impedance amplifier with programmable input impedance that hasbeen adjusted to match to an impedance of a photo-diode outputtransmission line external to the integrated circuit chip.
 14. Theintegrated circuit chip of claim 13, wherein the programmable inputimpedance has a predetermined minimum value and a predetermined maximumvalue that ranges to cover variations of a transmission line impedance.15. The integrated circuit chip of claim 13, wherein the open-gaintrans-impedance amplifier comprises a common gate amplifier stagefollowed by two common source amplifier stages, and wherein a last,stage of the open-gain trans-impedance amplifier provides voltage levelshifting for an output single and for a common mode signal of theopen-gain trans-impedance amplifier, and wherein the open-gaintrans-impedance amplifier comprises a low-pass filter to feed the commonmode signal to a bias voltage of a first amplifier stage of theopen-gain trans-impedance amplifier, and wherein low-pass filtering isomitted for the output signal of the open-gain trans-impedanceamplifier.